Are you talking about a fish tank? Or where is your reef? Hope this helps:
If it's a wreath, add ornaments, floral sprigs from the craft store and a giant bow.
I would go to the dollar store and buy inexpensive pine combs, small bulb ornaments and you'll find other Christmas items as well! I'd pick colors that go with the other decor.
There's a few ideas here for you Nan! I just decorated a large wreath for my garage! I found floral picks, ribbon, and Christmas bulbs at Hobby Lobby, JoAnn's, and the Dollar Store! I also picked up a few things at my local Goodwill! It's fun and easy to decorate a large wreath! Mine is an artificial one that I store from year to year. I used a blue and silver snowflake theme so it could be up all winter vs a Christmas theme.
Sometimes a visual helps. Here are some You Tube tutorials.
And you can get more ideas here:
here is some inspiration from hometalk
There are some cute ideas here. You could even spray paint pine cones various colors and decorate the wreath with those. https://www.digsdigs.com/50-awesome-christmas-wreaths-ideas-for-all-types-of-decor/
You could add flocking to the wreath, and decorate with ribbon and berries.